Hi guys,

I was wondering what octane rating you are using? The instruction book recommends 93/98(RON) and I only use 100(RON) (which is equivalent to around 95 in the US), but Does anyone have any experience how much difference it actually makes? The price difference here in Denmark is more than $0.20 pr liter or almost a dollar pr gallon between the two grades.

There was a test here in Denmark recently showing that a Kia Stinger 3.3 with similar power to the C43/450 gained around 15 hp average over three runs, which is hardly worth the extra 15% cost. But what is your experience with different fuel grades? Any noticeable difference?

 

Fortunately, where I live, we have 93 octane everywhere. I haven't run above 93, but would love to.

In terms of dropping below 91 (the minimum rating), the car's fuel economy and performance will start to drop. You're going to be more prone to engine knocking. Start to do that for a long period of time under normal loads or just for a short period of time under very heavy loads and you could damage the pistons, valves, and spark plugs.

The car's ECU will try to adjust for the lower-octane gas as much as possible, in order to prevent these sorts of things from happening, but it can't work miracles.

(15 horsepower is not insubstantial, btw; it's about 1 tenth of a sec worth or performance.)

 

FWIW, I've done multiple track days (same track) in both a C43 and C63 S with both 94 (R+M)/2 octane [98 RON] (no alocohol %) and 92 (R+M)/2 octane [96 RON] (up to 10% alcohol) with "pure" 92 and 94 sessions as well as about 50/50 +/- 10% mixes due to mid-day fills. I've not been able to perceive any performance or engine-temperature differences between sessions using these two fuels. That could be because my butt-dyno needs more sensitivity and/or because the ECU didn't have time to adapt timing ... or because there really wasn't any difference.

One important consideration is the quality of the fuel -- here I'm thinking age and cleanliness. In North America, it's much more likely that extra-premium-priced 94 octane will sit for a long time at most stations and may also accumulate more sediment due to low sales. Stations located near race tracks are likely to be exceptions to this. Something to consider when buying fuel from any station.

Only loosely related, on an extended highway trip in the C43 I filled the car with 91 (R+M)/2 octane [95 RON] then realized it was also labelled as "up to 20% alcohol." Within minutes it was very evident that the car was signficantly down on power and, given that I track fuel mileage using an app, ultimately recorded a significant decrease in fuel mileage. The data from 3 fill-ups is (Imperial gallons):
504 km - 35.24 mpg
590 km - 27.54 mpg (this was the 20% alcohol fuel)
310 km - 33.05 mpg
The last fill had 10-15% in-city driving but the first 2 fills were for 95+% highway driving.

 

We don’t get anything below 91 AKI here in Denmark. You can get 89 in a few places and the car will run on it, but with significantly limited performance due to the timing retarding. I’ve done my fair bit of ECU tuning in my youth, so I know the principles. I am more wondering if anyone has some first hand experience in comparing the two fuels.

The engine is designed to run on 91 AKI (requierment from the EU) but Mercedes claims it is optimized for 98 RON / 93 AKI. I’ll do some logging of timing advance and MAF readings and then I’ll switch to 95 RON for a period and compare the two to get some more details on it. I might even take it on the dyno to see if there really is a difference. One thing is for certain, 0.1 second isn’t worth an extra 15% on my fuel bill.

I’ve done some testing on two previous cars (a 325 hp Opel Insignia OPC and a BMW M135i) the BMW couldn’t care less, there was no timing advance and no reduction in airflow, thus no increased performance and no reduced fuel consumption. The Opel had a very significant difference in both so we dynoed it and there was about 30 hp difference, but it was on different days and it was just single runs, so the numbers can’t be trusted completely. Both were recommended 98 RON, and the fuel was 99 RON which was the best stuff you can get here (Germany has a 102 RON fuel for the most demanding engines). My suspicion is that the Opel was at it’s limits, therefore the fuel made more of a difference. You could even here the difference when the engine was cold.

So all that is left really, if the merc is more like the BMW than the Opel (which I expect) is the better additive package of the premium fuel, and really, then I’d rather spent a few bucks on a high quality aftermarket additive.

 

Thanks for the input! This comfirms my expectations. We were in Norway with the car, leaving Denmark on 100 RON but refueling on 95 RON in Norway, and I couldn’t feel any difference, but it turns out the car had a leaking intercooler at the time so it wasn’t producing full boost, and therefore I’m not really trusting that “datapoint”.

 

Don't be a schmuck, just look for acceptable fuel quality, preferably with 0% ethanol, and be on your merry way.

Unless racing with a tuned ECU optimized for E85, you are wasting your time and money with anything else -- especially for city driving.

 

Interesting you bring up the question of ethanol. Here on the West Coast both Shell and Chevron are known to run very high percentages of ethanol (in everything but 94) despite all their marketing - pretty surefire way to destroy your cats. Esso / Mobil appear better.